1. Field of the Invention
The present invention relates to periodic cleaning of the heating surfaces of a steam generator and particularly to the mechanical removal of deposits from such surfaces. More specifically, this invention is directed to a vibrational cleaning method wherein the interval between or the duration of successively performed cleaning steps is determined as a function of past experience and the level of contamination and thus is continuously adapted to the current need for cleaning. Accordingly, the general objects of the present invention are to provide novel and improved methods of such character.
2. Description of the Prior Art
Heat transfer surfaces of steam generators, the outside of coiled tubes for example, are typically exposed to flue gases which may contain particulate matter which adheres to such surfaces. Heat transfer efficiency is adversely affected by such deposits of particulate matter. There are several known techniques for cleaning steam generator heating surfaces including various chemical treatment processes, "soot blowing" and inducing vibrations in the steam generator which cause the particulate matter to be mechanically separated from the surfaces.
In the case of vibrational cleaning, it is known to employ an actuator, for example a pair of vibration inducing coils, to impart motion to the coils of the steam generator, for example at the resonant frequency thereof.
The mechanical excitation of the device to be cleaned, i.e., a steam generator coil, has also been accomplished by means of a fluidic actuator, a compressed air cylinder and associated coil engaging member for example, which operates periodically over a period of time. The vibration process is repeated after a particular cleaning interval, the interval and the time during which the actuator is periodically energized comprising a cleaning cycle. In the prior art, the parameters of the cleaning cycle have customarily been preset, by means of clock mechanisms for example, and were not easily or automatically varied.
The degree of dirt accumulation on the tubes of a steam generator is dependent upon several factors including the chemical composition of the particulate matter entrained in the flue gas, the load on the steam generator and the flue gas temperature. Thus, the rate of contamination, and thus the rate at which heat exchange efficiency diminishes, is not constant. Accordingly, the prior art vibrational cleaning techniques, which employ fixed times for the cleaning cycle, were not satisfactory. It is also disadvantageous, but nevertheless the conventional prior art practice, to perform tests to determine the cleaning intervals for each mode of operation of the steam generator. This requires, at minimum, a very long break-in period while the tests are performed at all of the various combinations of expected operating conditions. Further, since the operating conditions, and thus the amount of dirt accumulation, can change over the course of time, cleaning in accordance with the originally determined cleaning intervals may not optimize the efficiency of operation of the steam generator and time consuming further tests and adjustments will often be required.
As described in published German Patent Application No. 2,036,061, attempts have been made to regulate the cleaning intervals as a function of temperature measurements. For this purpose, the temperature downstream of a heat exchanger coil of the steam generator or at the steam generator flue gas exit have been monitored. The exercise of control over vibrational cleaning as a function of measured temperature has, however, proven to be unsuitable in actual operation because the temperatures measured are dependent upon several operating parameters and do not allow any clear conclusion as to the degree of contamination of the heating surfaces. It is also to be observed that the temperature measurements themselves have been somewhat suspect, thus lessening the reliability of controlling vibrational cleaning as a function of monitored temperature, because of contamination of the temperature measuring devices themselves by exposure to a rather harsh operating environment.